Enhanced oil recovery profile control with crosslinked anionic acrylamide copolymer gels

ABSTRACT

An aqueous polymeric gel-forming composition capable of selectively plugging highly permeable zones in subterranean oil-bearing formations. The composition comprises an aqueous solution of an anionic acrylamide copolymer of high molecular weight, comprising about 5 to 95 weight percent of 2-acrylamido-2-methylpropanesulfonic acid, about 2 to 95 weight percent of N-vinyl-N-methyl acetamide and about 5 to 93 weight percent of acrylamide, and a crosslinking agent selected from the group consisting of transition metal ions, phenolic resins and amino resins. The compositions of this invention will form stable gels in brines of wide-ranging salinity and are effective at the pH levels encountered in carbon dioxide and water flooding operations. Also provided is a process for selectively plugging regions of higher permeability within an oil-bearing formation to improve sweep efficiency during a fluid flood oil recovery operation.

This is a division of copending application Ser. No. 07/595,721, filedon Oct. 9, 1990 now U.S. Pat. No. 5,079,278, which is acontinuation-in-part of application Ser. No. 07/453,241, filed on Dec.13, 1989, abandoned, which is a continuation-in-part of application Ser.No. 07/283,398, filed on Dec. 12, 1988, abandoned.

FIELD OF THE INVENTION

This invention relates to gel-forming crosslinked compositions which areuseful in the control of permeability in subterranean oil-bearingformations. Use of these gels in the oil recovery process of thisinvention can yield improved sweep efficiency during fluid floodingoperations. More particularly, this invention relates to the use ofsynthetic, high molecular weight, anionic acrylamide copolymers to formthe crosslinked gel compositions of this invention.

BACKGROUND OF THE INVENTION

In the production of oil from subterranean formations, it is usuallypossible to recover only a small fraction of the total oil present inthe formation by so-called primary recovery methods which utilize onlythe natural forces present in the reservoir. To recover oil beyond thatwhich is produced by primary methods, a variety of supplementalproduction techniques have been employed. In these supplementaltechniques, commonly referred to as secondary recovery operations, afluid is introduced into the oil-bearing formation in order to displaceoil to a production system comprising one or more production wells. Thedisplacing or "drive" fluid may be an aqueous liquid such as brine orfresh water, a gas such as carbon dioxide, steam or dense-phase carbondioxide, an oil-miscible liquid such as butane, or an oil andwater-miscible liquid such as an alcohol. Often, the most cost-effectiveand desirable secondary recovery methods involve the injection of anaqueous or carbon dioxide flooding medium into an oil-bearing formation,either alone or in combination with other fluids. In practice, a numberof injection and production wells will be used in a given field arrangedin conventional patterns such as a line drive, five spot or invertedfive spot, seven spot or inverted seven spot.

In the use of the various flooding techniques, it has become a commonexpedient to add various polymeric thickening agents to the drive fluidto increase its viscosity to a point where it approaches that of the oilwhich is desired to be displaced, thus improving the displacement of oilfrom the formation. The polymers used for this purpose are often said tobe used for "mobility" control.

Another problem encountered is that certain injected drive fluids may bemuch lighter than the reservoir fluids and thus separate by gravity,rising toward the top of the flowing region and resulting in thebypassing of the lower regions. This phenomenon is known as gravityoverride.

Also encountered in the use of various flooding techniques is asituation caused by the fact that different regions or strata havedifferent permeabilities. In this situation, the drive fluidpreferentially enters the regions of higher permeability due to thelower resistance to flow present rather than the regions of lowpermeability where significant volumes of oil often reside.

It therefore is often desirable to plug the regions of highpermeability, or "thief" zones, either partly or entirely, so as todivert the drive fluid into regions of lower permeability. Themechanical isolation of these thief zones has been tried but verticalcommunication among reservoir strata often renders this methodineffective. Physical plugging of the high permeability regions bycements and solid slurries has also been tried with varying degrees ofsuccess; however, these techniques have the drawback thatstill-productive sites may be permanently closed.

As a result of these earlier efforts, the desireability of designing aviscous slurry capable of sealing off the most permeable layers so thatthe drive fluid would be diverted to the underswept, "tighter" regionsof the reservoir, became evident. This led to the use of oil/wateremulsions, as well as gels and polymers for controlling the permeabilityof the formations. This process is frequently referred to as "profile"control, a reference to the control of the vertical permeability profileof the reservoir. Profile control agents which have been proposedinclude oil/water emulsions, gels, e.g., lignosulfate gels and polymers,with polymers being the most extensively applied in recent years.

Of the secondary and tertiary enhanced oil recovery processes,waterflooding, carbon dioxide flooding, miscible or immiscible gasflooding and steam flooding are of particular interest and importance.As indicated, profile control can often improve performance in suchprocesses by reducing the effect of permeability inhomogeniety orstratification and gravity override. A gel suitable for profile controlmust be stable enough to continue to impede flow for long periods oftime at the given temperature, salinity and pH of a particularoil-bearing reservoir. A gel must also have adequate mechanical strengthto resist the pressures which will be applied during the subsequent oilrecovery flooding step. There are a variety of materials commerciallyavailable for profile control, all of which perform differently and havetheir own, often unique limitations.

Among the many polymers examined thus far are polyacrylamides,polysaccharides, celluloses, furfural-alcohol and acrylic-epoxy resins,silicates and polyisocyanurates. Proposals have been made for the use ofinorganic polymers, especially inorganic silicates, as permeabilitycontrol agents. For example, U.S. Pat. Nos. 4,009,755 and 4,069,869disclose the use of inorganic silicates for this purpose. In thepermeability control method described in these patents, an organicpolymeric permeability control agent such as a crosslinkedpolyacrylamide or polysaccharide is first injected into the reservoir,followed by an aqueous solution of an alkaline metal silicate and amaterial that reacts with the silicate to form a silicate gel whichplugs the high permeabiality regions in the formation. An alkaline metalsilicate is typically used as the source of silica and the gelling agentis usually an acid or acid-forming compound such as a water solubleammonium salt, a lower aldehyde, an aluminum salt or an alkaline metalaluminate. The problem, however, with many inorganic silicates is thattheir solutions are often quite viscous and stable only under alkalineconditions. As soon as conditions become acidic, a silicate gel isformed. Although this is the desired reaction for plugging theformation, it may occur prematurely.

Other attempts have been made to achieve profile control. One suchattempt is described in U.S. Pat. No. 4,498,539 to Bruning, whichdiscloses delayed gelable compositions for injection of a waterthickening amount of a polymer capable of gelling in the presence of acrosslinking agent so that after the composition has penetrated into anunderground formation and positioned itself preferentially in the highlypermeable strata, the delayed gelation is triggered by in-situhydrolysis of an ester which reduces the pH of the composition to thegelable range thereby producing in-depth plugging of the strata with thegelled polymer.

A group of polymeric thickeners which has been used in waterfloodingoperations is the xanthan polysaccharides. Xanthan polysaccharides areproduced by the action of bacteria of the genus Xanthomonas oncarbohydrates. For example, U.S. Pat. Nos. 3,757,863 and 3,383,307disclose mobility control by the use of polysaccharides in the presenceof polyvalent metal ion crosslinking agents. U.S. Pat. No. 3,810,882discloses the possibility of using certain reducible complex metal ionsas cross-linking agents for polysaccharides. U.S. Pat. Nos. 4,078,607and 4,104,193 describe a method for improving the efficiency ofwaterflooding operations by a particular polysaccharide prehydrationtechnique. U.S. Pat. No. 4,413,680 describes the use of crosslinkedpolysaccharides for selective permeability control in oil reservoirs.

U.S. Pat. No. 3,908,760 describes a polymer water-flooding process inwhich a gelled, water-soluble Xanthomonas polysaccharide is injectedinto a stratified reservoir to form a slug, band or front of gelextending vertically across both high permeability and low permeabilitystrata. This patent also suggests the use of complexed polysaccharidesto block natural or man-made fractures in formations. The use ofpolyvalent metal ions for crosslinking xanthan polysaccharides and otherpolymers which are to be used for permeability control is described inU.S. Pat. Nos. 4,009,755, 4,069,869 and 4,413,680. The use ofphenol/aldehyde crosslinking agents with xanthan polysaccharides andother polymers is disclosed in U.S. Pat. Nos. 4,323,123 and 4,440,228.

Another type of polysaccharide which has been experimented with in thearea of profile control is the non-xanthan, heteropolysaccharide S-130.S-130 belongs to the group of non-xanthan welan gums. S-130 is producedby fermentation with a microorganism of the genus Alcaligenes. Anotherwelan gum heteropolysaccharide, known as S-194, is also produced byfermentation with a microorganism of the genus Alcaligenes. U.S. Pat.No. 4,658,898 discloses the use of welan gum S-130 in saline waters.Crosslinking with trivalent cations, such as chromium, aluminum,zirconium and iron is also disclosed. Additionally, crosslinking withorganic compounds containing at least two positively charged nitrogenatoms is disclosed in U.S. Pat. No. 4,658,898.

U.S. Pat. No. 4,787,451, the inventor of which is also the inventor ofthe present invention, discloses the use of melamine-formaldehyde andother amino resins to crosslink various polymers. U.S. Pat. No.4,787,451 is hereby incorporated by reference in its entirety.

A major part of the work conducted in the area of profile control hasdealt with the use of polyacrylamides. Polyacrylamides have been usedboth in their normal, non-crosslinked form as well as in the form ofcrosslinked metal complexes, as described, for example, in U.S. Pat.Nos. 4,009,755, 4,069,869 and 4,413,680. Shear degradation duringinjection and sensitivity to reservoir brines tend to diminish thebeneficial effects derived from these polyacrylamides.

U.S. Pat. No. 4,246,124 discloses gelled compositions suitable asfracture fluids and water diversion agents in which a polymericviscosifier selected from a group including the polyacrylimides iscrosslinked with small amounts of phenolic and aldehyde compounds. Theuse of the monomer 2-acrylamido-2-methyl-propanesulfonic acid (AMPS)®and its alkali metal salts for copolymerization with acrylamide ormetacrylamide are disclosed therein and form a preferred group ofcopoolymers. These copolymers, when used in the examples of U.S. Pat.No. 4,246,124, were used at a concentration of 1% to achieve the desiredwater thickening effect.

U.S. Pat. No. 4,579,667 discloses the use of an anionic partiallyhydrolyzed polyacrylamide together with a water-soluble cationicpolyamide-epichlorohydrin resin to from gelled aqueous compositions inbrines having salt concentrations of 1 to 10% at a pH of about 3 to 11.Suitable anionic polyacrylamides disclosed therein include any partiallyhydrolyzed homopolymer of acrylamide, homopolymers of methacrylamide andcopolymers of acrylamide or methacrylamide with other water-solublevinyl addition monomers containing or capable of generating an anioniccharge.

U.S. Pat. No. 4,785,028, a co-inventor of which is also the inventor ofthis invention, discloses aqueous gels formed by partially hydrolyzingan aqueous solution of a terpolymer comprising 5 to 95% by weight of theAMPS® monomer, 5 to 95% by weight of N-vinyl-N-methyl acetamide (VMA)and 0 to 80% by weight of acrylamide and thereafter crosslinking theresultant hydrolyzed polymer intermediate with transition metal ions,melamine/formaldehyde resin or resorcinol/formaldehyde resin. Theaforementioned partial hydrolysis is conducted by refluxing the polymerwith an alkali metal hydroxide in an amount of about 0.5 to 5.0% byweight for about 7 to 16 hours at a temperature of about 100° C. to formthe polymer intermediate. U.S. Pat. No. 4,785,028 is incorporated byreference in its entirety. The terpolymers used in U.S. Pat. No.4,785,028 have a relatively low weight-average molecular weight,generally about 0.8 to about 1.0×10⁶, as compared with the polymersuseful in the practice of this invention. As such, high terpolymerconcentrations are required to practice the invention of U.S. Pat. No.4,785,028.

As demonstrated, there are a variety of materials, many of which arecommercially available, which have some utility in profile control. Asis known, a gel suitable for profile control must be stable enough tocontinue to impede flow for long periods of time. This requires not onlythat the gel formed by the polymer should be stable enough to withstandthe relatively high temperatures encountered in some reservoirs--initself, a difficult requirement--but also that the gel should be stableover as wide a range of pH conditions as possible so that the polymerwill have the potential of being used in reservoirs of different kinds,e.g. sandstone, carbonate rock and others. Stability to various oilfieldbrines is another desirable requirement. Many of the known types oforganic gel forming polymers are unsatisfactory in one respect oranother, e.g. temperature stability, brine stability, pH range, so thatthere has been a continuing need for new and different polymers forpermeability control purposes. Biopolymers such as xanthan gums areunstable above about 140° F. Synthetic polyacrylamides, depending uponthe nature and amount of other functional groups such as alkyl sulfonateor pyrrolidone, as well as other factors, will have a temperature abovewhich they will not be useful at a given salinity. Certainpolyacrylamides, such as those disclosed in U.S. Pat. No. 4,785,028require high dosages and a time-consuming hydrolysis step to form usefulpolymeric gels.

Therefore, what is needed is an economical gel which can be used forprofile control during enhanced oil recovery under the harsh conditionsencountered in a subterranean formation.

It is therefore an object of this invention to provide for a new use ofa known polymer heretofore utilized as a viscosifier of water-basedfluids.

It is another object of this invention to provide a crosslinkedacrylamide copolymer gel suitable for profile control in enhanced oilrecovery operations.

It is a yet further object of this invention to provide for asubstantially stable gel when high temperatures and/or low pH's areencountered in a reservoir.

It is a yet still further object of this invention to provide for agelation reaction which will proceed in a reservoir environment of anylevel of salinity.

It is yet another object of this invention to provide for an economicalgel for use as a profile control agent during enhanced oil recoveryoperations.

It is still yet another object of this invention to provide a processfor selectively plugging regions of higher permeability within anoil-bearing subterranean formation to obtain improved sweep efficiencyduring a fluid flood oil recovery operation.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a gel-formingcomposition and a process for utilizing the gel-forming composition tocontrol the permeability of subterranean, oil-bearing formations. Ananionic acrylamide copolymer of high molecular weight, comprising about5 to 95 weight percent 2-acrylamido-2-methylpropanesulfonic acid; about2 to 95 weight percent of N-vinyl-N-methyl acetamide; and about 5 to 93weight percent of acrylamide, wherein the copolymer has a weight-averagemolecular weight of at least 5×10⁶, is crosslinked with an agentselected from the group consisting of transition metal ions, phenolicresins and amino resins to form an aqueous gel. Such a gel is useful inwater flooding and carbon dioxide flooding oil recovery operations whereimproved sweep efficiency is desired. For example, a gel-formingcomposition of this invention may be injected into a region of higherpermeability within an oil-bearing subterranean formation to selectivelyplug this more permeable region and thus improve the sweep efficiency ofa subsequent fluid flooding operation. The gels which are formed arestable under the harsh conditions often encountered when producingfluids from a subterranean formation during enhanced oil recoveryoperations.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In preparing the gel-forming composition of this invention, an anionicacrylamide copolymer of high molecular weight is used which is producedby copolymerizing at 0° to 130° C., an aqueous mixture of monomerscomprising about 5 to 95 weight percent2-acrylamido-2-methylpropanesulfonic acid; about 2 to 95 weight percentof N-vinyl-N-methyl acetamide; and about 5 to 93 weight percent ofacrylamide. The well-known Trommssdorff-Norrish method ofcopolymerization may be utilized and the resultant copolymer has aweight-average molecular weight of at least 5×10⁶. Copolymers producedin the aforementioned manner having weight-average molecular weights ofat least 7×10⁶ are more preferred. A method for making kindred polymersis disclosed in U.S. Pat. No. 4,521,579, incorporated by reference inits entirety.

It is to be understood that whenever reference is made within thisdisclosure to the term "molecular weight", it is the weight-averagemolecular weight that is being made reference to, rather than thenumber-average molecular weight. A suitable and preferred method fordetermining weight-average molecular weight is the light-scatteringphotometer method discussed at pages 75-84 of the "Textbook of PolymerScience, Second Edition," by F. W. Billmeyer, Jr., published by WileyInterscience, John Wiley and Sons, Inc., New York (1971), the contentsof which are incorporated by reference in their entirety. A copolymerpreferred for use herein is marketed by American Hoechst Corporation,located in Houston, Tex., and is sold under the trade name ®Hostamer V3140. This product is sold as a viscosifier of water-based fluids havingmedium to high salinity. The copolymer is derived from, as is preferred,about 40% by weight of AMPS®, 30% by weight of acrylamide, and 30% byweight of N-vinyl-N-methyl acetamide. Said copolymer is believed to havea weight-average molecular weight of about 7 to 9 million.

To obtain an aqueous solution of the preferred copolymer, the copolymershould be added slowly to water with vigorous stirring. At 25° C., 0.5 gof the copolymer will dissolve in 100 ml of fresh water (hardness of 90mg CaO per liter). The manufacturer states that faster and easierdissolving can be achieved by the previous wetting of the granules withsmall amounts of alcohol, such as isopropanol. By preparing the solutionin this manner, the insoluble residue formed is less than 0.2% byweight.

Crosslinking agents useful in the practice of this invention aretransition metal ions, phenolic resins and amino resins. Suitablecrosslinking agents include polyvalent metal cations such as Al⁺³, Cr⁺³,Fe⁺³, Sb⁺³ and Zr⁺⁴. Also suitable for crosslinking are multifunctionalamines such as diamines. For example, aluminum citrate can be admixedwith the polymer or in slugs alternating with polymer slugs. Solublecompounds of Cr⁺³ or Fe⁺³ can be used, or oxidizable compounds ofdivalent iron such as FeCl₂ can be used in conjunction with an oxidant.

In the practice of this invention, a pre-formed phenolic resin can beused; said resin generally obtained by the condensation of phenol orsubstituted phenols with an aldehyde such as formaldehyde, acetaldehydeand furfural. Additionally, the phenol and aldehyde constituents can beadded separately to produce the compositions of this invention, ratherthan being added as a pre-formed phenolic resin.

Any suitable water-dispersible phenol can be used in the practice ofthis invention. Phenolic compounds suitable for use in the presentinvention include phenol, resorcinol, catechol, 4,4'-diphenol,1,3-dihydroxynaphthalene, pyrogallol, phloroglucinol and other similarcompounds. Resorcinol and phenol are the preferred phenolics for mostwater and carbon dioxide drive applications, with resorcinol beingparticularly preferred. The choice of a phenol compound will be basedlargely on the rate of gelation desired. Mixtures of the named phenolsare also useful.

A broad range of water-dispersible aldehydes are useful in the practiceof the present invention. Both aliphatic and aromatic monoaldehydes anddialdehydes can be used. The useful aliphatic monoaldehydes includethose containing from one to ten carbon atoms per molecule, such asformaldehyde, paraformaldehyde, acetaldehyde, proprionaldehyde,butylaldehyde, isobutylaldehyde, heptaldehyde and others. Among theuseful dialdehydes are glyoxyl, glutaraldehyde and terephthaldehyde.Mixtures of the various, aforementioned aldehydes are also useful in thepractice of the present invention. Of the preferred aldehyde compounds,formaldehyde is particularly preferred.

Amino resins may either be preformed resins, such as the preferredmelamine/formaldehyde resins, mixtures of amino compounds and aldehydecompounds or mixtures of preformed resins and aldehyde compounds. Theaforementioned aldehyde compounds are also useful in the amino resincrosslinking agents of this invention. Particularly preferred aminoresins are disclosed in U.S. Pat. No. 4,787,451, which is incorporatedby reference in its entirety.

Of the transition metal ions useful in the practice of this invention,Cr⁺³ ions are particularly preferred for forming gels. Chromic nitrateand chromic chloride have been utilized to form gels. The pH mayoptionally be adjusted before crosslinking. Redox systems such as sodiumdichromate and sodium bisulfite have been utilized to obtain Cr⁺³ ions.Similar redox systems are described in U.S. Pat. No. 3,749,172, which ishereby incorporated by reference. When forming these gels, Cr⁺³ ions areused in a preferred amount of from about 50-750 ppm. As is understood bythose skilled in the art, the amount of Cr⁺³ ions, or other transitionmetal ions, utilized will vary depending upon the molecular weight ofthe particular polymer utilized. In any event, the metal ions should bepresent in an amount sufficient to obtain the desired gelling effect.

Gels resultant from crosslinking the anionic acrylamide copolymer areformed in a preferred range of between about pH 5 and pH 8 when formingthese gels with Cr⁺³, and in a preferred range between about pH 3 and pH10 with other crosslinking agents. These gels can be formed in freshwater, distilled water, saline water and synthetic sea water. The amountof organic crosslinking agent useful in the practice of this inventionwill generally be a small but effective amount sufficient to initiateand cause gelation of an aqueous solution of the acrylamide copolymer.It will generally be found that the amount of amino or phenolic resinuseful to form advantageous gels will be in the range of 0.02 to 5.0weight percent. When preformed resin is not employed, the amount of theamino or phenol compound used will be in the range of 0.01 to about 2.0weight percent, with concentrations of 0.05 to 1.0 weight percentpreferred. The concentration of aldehyde used will be in the range of0.01 to 3.0 weight percent, with concentrations of 0.1 to 1.0 weightpercent preferred.

The amount of the acrylamide copolymer useful in preparing the aqueouscrosslinked gels of this invention may vary depending on the particularcopolymer used, its purity and the desired properties of the resultantgels. Generally speaking, the quantity of the copolymer used will be awater-thickening or viscosifying amount, that is, an amount which willsignificantly increase the viscosity of the water to which it has beenadded. Amounts ranging from about 0.05 to about 1 weight percent arepreferred, with amounts ranging from about 0.1 to about 0.5 weightpercent particularly preferred. The low concentrations required to formsuitable gels are believed to be the result of the high molecular weightof these copolymers.

Temperatures from about ambient to about 250° F. can be used to formthese gels. As the temperature increases, gels form faster. Whenutilized for profile control in waterflooding and carbon dioxideenhanced recovery operations, the gels may be formed in-situ. Whenemployed in said enhanced recovery operations, the gels have exceptionalstability at the temperatures, salinities, pH's and pressuresencountered in the subterranean formation.

Contrary to the disclosure of U.S. Pat. No. 4,785,028, it has been foundthat no partial hydrolysis step is required to form the polymer gelsdescribed herein. While it is not known why this step is not required informing the gels of this invention, it is thought that it may be relatedto the copolymer's higher molecular weight or that perhaps somehydrolysis takes place when the gels are formed as disclosed herein.

The aqueous crosslinked gel compositions and the process for selectivelyplugging regions of higher permeability within oil-bearing formationsaccording to the present invention can be used in conjunction with thoseflooding operations in which a flooding fluid, usually water or carbondioxide, is injected into a formation through injection wells whichextend from the surface of the earth into the formation.

The following data demonstrate the extent of the unexpected beneficialresults obtained with the crosslinked polymeric gels of the presentinvention. The invention is illustrated by the following non-limitingexamples:

EXAMPLES 1-10

A once-percent stock solution of Hoechst ®Hostomer V 3140 anionicacrylamide copolymer was prepared in accordance with the aforementionedrecommendations of the manufacturer prior to preparing the gel solutionsof Examples 1-10. As shown in Table 1 below, three types of crosslinkingagents were utilized. The transition metal ion crosslinking agent usedwas Cr⁺³, prepared in the well-known manner from Cr(NO₃)₃. The aminoresin used was Parez 613, a commercially available 3:1 mole ratiocondensate of formaldehyde and melamine. This resin can be purchasedfrom American Cyanamid Co. of Wayne, N.J. The phenolic crosslinkingagents used were various blends of commercially available resorcinol andformaldehyde.

The gels of Examples 1-6 were not subjected to a partial hydrolysis stepprior to crosslinking. The gels of Examples 7 and 8 were formedfollowing the aging of the 1% copolymer stock solution in sealedampoules for 5 days at 210° F. This step is known to cause partialhydrolysis of the copolymer. The gels of Examples 9 and 10 were formedfollowing the subjection of the 1% copolymer stock solution to a partialhydrolysis step. This step was accomplished by refluxing the stocksolution of V 3140 in 0.5M NaOH for 15.5 hours, followed by cooling andneutralizing with concentrated HCl.

All gel solutions were prepared in distilled water (total dissolvedsolids (TDS)=0). The pH of the various solutions was adjusted tosimulate either a water flooding application (pH of about 5 to 7) or acarbon dioxide flooding application (pH of about 3.5). Solutions of 0.1NNaOH or 0.1N HCl were used to adjust the pH to the desired level. Thegels were stored at 175° F. for periods ranging from 16 to 19 weeks.Results are presented in Table 1.

                                      TABLE 1                                     __________________________________________________________________________    Gel Stability at 175° F.                                               All Gels Prepared in Distilled Water                                          Test Duration: 16-19 Weeks                                                              Crosslinking Agent Concentration                                              Cr.sup.+3                                                                         Amino                                                                             Phenolic Resin                                              Copolymer.sup.1                                                                         Ions                                                                              Resin.sup.2                                                                       Resorcinol                                                                          HCHO                                                  Conc, PPM (PPM)                                                                             (PPM)                                                                             (PPM) (PPM)                                                                             pH Comments                                       __________________________________________________________________________    1  5000   440   0 0     0   5.45                                                                             Loose gel                                      2  6000   0   8400                                                                              0     0   3.55                                                                             Stiff gel                                      3  6300   0   8360                                                                              0     0   6.87                                                                             Loose gel                                      4  1900   0   3300                                                                              0     0   3.60                                                                             Loose gel                                      5  5200   0     0 2000  2800                                                                              3.50                                                                             Stiff gel                                      6  4000   0     0 1000  1850                                                                              3.40                                                                             No syneresis                                   7  .sup. 5000.sup.3                                                                     460   0 0     0   6.20                                                                             Firm gel                                       8  .sup. 5000.sup.3                                                                     0   4000                                                                              0     0   3.50                                                                             No synersis                                    9  .sup. 4750.sup.4                                                                     0   2480                                                                              0     0   6.60                                                                             No synersis                                    10 .sup. 5600.sup.4                                                                     0     0 1785  2140                                                                              3.30                                                                             No synersis                                    __________________________________________________________________________     .sup.1 Hoechst ® Hostamer V 3140 anionic acrylamide copolymer.            .sup.2 Parez 613, 3:1 mole ratio condensate of formaldehyde/melamine.         .sup.3 Copolymer solution aged in glass ampoule for 5 days at 210°     F. prior to forming gel and storing at 175° F. Procedure causes        partial hydrolysis.                                                           .sup.4 Copolymer solution subjected to partial hydrolysis.               

As indicated, thermal and hydrolytic gel stability was found to beexcellent in all cases. Additionally, these examples show that the highmolecular weight copolymer can be used for profile control inwaterflooding (gels 1, 3, 7 and 9) and CO₂ -flooding (gels 2, 4, 6, 8and 10) operations. Stiff gels (2 and 5) and loose gels (1, 3 and 4) canbe made. Very low concentrations can be used; gel 4 was made with only1900 ppm of copolymer. While not found to detract in any way from gelstability, as shown by the excellent results obtained using any of thecrosslinking agents, partial hydrolysis is not required to form thenovel gels of this invention.

EXAMPLES 11-22

The anionic acrylamide copolymer ®Hostamer V 3140 was dispersed asbefore in the various brines identified in Table 2, below. A full matrixof samples of 3000 ppm V 3140, 1000 ppm resorcinol and 1850 ppmformaldehyde were prepared in synthetic sea water (3% salinity) and 6,12 and 23% brines (90% NaCl and 10% CaCl₂). No partial hydrolysis stepwas utilized prior to solution preparation. The pH level used for allsamples was 3.5. The samples prepared were stored for evaluation at 140,175 and 210° F. Results are as follows:

                  TABLE 2                                                         ______________________________________                                        Gel Stability at pH = 3.5                                                     Each Gel Contains: 500 ppm Copolymer                                          1000 ppm Resorcinol and 1850 ppm Formaldehyde                                              Syneresis, in Percent, At                                             Salinity, Test    4                                                      Ex.  % TDS     Temp    Weeks 8 Weeks  12 Weeks                                ______________________________________                                        11   Sea Water 140     0      0       0                                       12    6        140     0      0       0                                       13   12        140     0      0       2                                       14   23        140     8     22       40                                      15   Sea Water 175     3      3       3                                       16    6        175     7     10       15                                      17   12        175     4     10       13                                      18   23        175     3      7       8                                       19   Sea Water 210     22    35       Discontinued                            20    6        210     10    25       Discontinued                            21   12        210     18    50       Discontinued                            22   23        210     30    Discontinued                                                                           Discontinued                            ______________________________________                                    

As shown, test results indicate that the gels of this invention providegood thermal and hydrolytic stability at temperatures of up to at least210° F. and at salinities ranging from sea water (TDS˜3%) to up to atleast a 23% TDS brine. For the samples stored at 140° F., virtually nosyneresis was observed for the gels prepared in solutions of ≦12%salinity. The 23% salinity gel began to synerese badly after 10 weeks.The gels stored at 175° F. were found to yield the best stability, withonly minor syneresis observed even at 23% salinity. Generally acceptablegels were obtained even at 210° F. Significant syneresis was observed at6-8 weeks, with testing dicontinued when the samples exceeded about 50%syneresis. It must be noted that while gels exhibiting low syneresis arethought to be preferred, it is not known whether the gels exhibitinghigh syneresis would be suitable for profile control. It is thought thateven the high syneresis gels may provide some utility in profile controloperations. While only phenolic crosslinking agents were used in thetests of gels at various salinities, it is believed that the resultsobtained and presented in Table 2 are translatable to the other organiccrosslinking agents disclosed herein.

Where it is desired to obtain increased sweep efficiency, gels of thisinvention can be used to plug a previously swept portion of a formation.These gels can be directed to areas of increased porosity by utilizationin any of the below methods, as well as others which those skilled inthe art will plainly recognize. Additionally, the permeability controltreatment may be carried out periodically when necessary to achieve thedesired permeability profile.

One method where gels of this invention can be utilized is during awaterflooding process for the recovery of oil from a subterraneanformation. After plugging the more permeable zones of a reservoir withthe novel gels of this invention, a waterflooding process can becommenced or resumed. U.S. Pat. No. 4,479,894, issued to Chen et al.,describes one such waterflooding process. This patent is herebyincorporated by reference in its entirety.

Steamflood processes which can be utilized when employing the gelsdescribed herein are detailed in U.S. Pat. Nos. 4,489,783 and 3,918,521issued to Shu and Snavely, respectively. These patents are herebyincorporated by reference herein.

Gels described herein can also be used in conjunction with a carbondioxide flooding process, either alone, or in conjunction with acyclical steam stimulation in a heavy oil recovery process to obtaingreater sweep efficiency. Cyclic carbon dioxide steam stimulation can becommenced or resumed after plugging the more permeable zones of thereservoir with the novel gels of this invention. A suitable process isdescribed in U.S. Pat. No. 4,565,249 which issued to Pebdani et al. Thispatent is hereby incorporated by reference in its entirety. Increasedsweep efficiency can be obtained when the subject gels are used incombination with a carbon dioxide process for recovering oil. Prior tocommencement or resumption of the carbon dioxide process, the morepermeable zones are plugged with these novel gels.

Although the present invention has been described with preferredembodiments, it is to be understood that modifications and variationsmay be utilized without departing from the spirit and scope of thisinvention, as those skilled in the art will readily understand. Suchmodifications and variations are considered to be within the purview andscope of the appended claims.

What is claimed is:
 1. A process for recovering oil from a subterraneanoil-bearing formation having relatively high permeability zones andrelatively low permeability zones penetrated by at least one injectionwell and at least one production well in fluid communication with asubstantial portion of the formation comprising the steps of:(a)injecting into the formation via the injection well an aqueous gelforming solution comprising water; a viscosifying amount of an anionicacrylamide copolymer of high molecular weight produced by copolymerizingan aqueous mixture of monomers comprising about 5 to 95 weight percent2-acrylamido-2-methylpropanesulfonic acid; about 2 to 95 weight percentof N-vinyl-N-methyl acetamide; and about 5 to 93 weight percent ofacrylamide, said copolymer having a molecular weight of at least 5×10⁶and able to form a stable crosslinked gel without first subjecting saidcopolymer to a partial hydrolysis step; and a crosslinking agentselected from the group consisting of transition metal ions, phenolicresins and amino resins, said crosslinking agent present in an amountsufficient to cause gelation of the aqueous solution of said anionicacrylamide copolymer of high weight-average molecular weight and producethe crosslinked gel; (b) injecting a flooding fluid into the formationvia the injection well that preferentially enters the low permeabilityzones; and (c) recovering fluids including oil from the formation viathe production well.
 2. The process of claim 1, wherein in step (a) saidcrosslinking agent is said transition metal ions.
 3. The process ofclaim 2, wherein in step (a) said transition metal ions are Cr⁺³ ions.4. The process of claim 3, wherein in step (a) said Cr⁺³ ions arepresent in an amount of about 50 to about 750 ppm.
 5. The process ofclaim 1, wherein in step (a) said crosslinking agent is an amino resin.6. The process of claim 5, wherein in step (a) said amino resin is acondensate of formaldehyde and melamine.
 7. The process of claim 6,wherein in step (a) said formaldehyde and melamine are present in a 3:1molar ratio.
 8. The process of claim 7, wherein in step (a) said aminoresin is present in an amount of 0.02 to 5.0 weight percent.
 9. Theprocess of claim 1, wherein in step (a) said crosslinking agent is aphenolic resin.
 10. The process of claim 9, wherein in step (a) saidphenolic resin comprises about 1 to about 99 weight percent of at leastone phenolic compound selected from the group consisting of phenol,resorcinol, catechol, phloroglucinol, pyrogallol, 4,4'-diphenol and1,3-dihydroxynaphthalene; and about 1 to about 99 weight percent of atleast one aldehyde component selected from the group consisting ofaliphatic monoaldehydes, aromatic monoaldehydes, aliphatic dialdehydesand aromatic dialdehydes.
 11. The process of claim 10, wherein in step(a) said phenolic compound is resorcinol and said aldehyde component isformaldehyde.
 12. The process of claim 10, wherein in step (a) saidphenolic resin is present in an amount of about 0.02 to about 5.0 weightpercent.
 13. The process of claim 1 wherein said copolymer is present inan amount of about 0.05 to about 1.0 weight percent.
 14. The process ofclaim 1 wherein said water is a brine.
 15. The process of claim 1,wherein in step (b) the flooding fluid comprises water.
 16. The processof claim 1, wherein in step (b) the flooding fluid comprises carbondioxide.
 17. The process of claim 1, wherein in said step (b) theflooding fluid comprises steam.